Re: [RFC PATCH] mm: filemap: fix nr_pages calculation overflow in filemap_map_pages()

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 3/13/26 1:14 PM, Dev Jain wrote:
> 
> 
> On 13/03/26 10:41 am, Dev Jain wrote:
>>
>>
>> On 13/03/26 9:15 am, Baolin Wang wrote:
>>> When running stress-ng on my Arm64 machine with v7.0-rc3 kernel, I encountered
>>> some very strange crash issues showing up as "Bad page state":
>>>
>>> "
>>> [  734.496287] BUG: Bad page state in process stress-ng-env  pfn:415735fb
>>> [  734.496427] page: refcount:0 mapcount:1 mapping:0000000000000000 index:0x4cf316 pfn:0x415735fb
>>> [  734.496434] flags: 0x57fffe000000800(owner_2|node=1|zone=2|lastcpupid=0x3ffff)
>>> [  734.496439] raw: 057fffe000000800 0000000000000000 dead000000000122 0000000000000000
>>> [  734.496440] raw: 00000000004cf316 0000000000000000 0000000000000000 0000000000000000
>>> [  734.496442] page dumped because: nonzero mapcount
>>> "
>>>
>>> After analyzing this page’s state, it is hard to understand why the mapcount
>>> is not 0 while the refcount is 0, since this page is not where the issue first
>>> occurred. By enabling the CONFIG_DEBUG_VM config, I can reproduce the crash as
>>> well and captured the first warning where the issue appears:
>>>
>>> "
>>> [  734.469226] page: refcount:33 mapcount:0 mapping:00000000bef2d187 index:0x81a0 pfn:0x415735c0
>>> [  734.469304] head: order:5 mapcount:0 entire_mapcount:0 nr_pages_mapped:0 pincount:0
>>> [  734.469315] memcg:ffff000807a8ec00
>>> [  734.469320] aops:ext4_da_aops ino:100b6f dentry name(?):"stress-ng-mmaptorture-9397-0-2736200540"
>>> [  734.469335] flags: 0x57fffe400000069(locked|uptodate|lru|head|node=1|zone=2|lastcpupid=0x3ffff)
>>> ......
>>> [  734.469364] page dumped because: VM_WARN_ON_FOLIO((_Generic((page + nr_pages - 1),
>>> const struct page *: (const struct folio *)_compound_head(page + nr_pages - 1), struct page *:
>>> (struct folio *)_compound_head(page + nr_pages - 1))) != folio)
>>> [  734.469390] ------------[ cut here ]------------
>>> [  734.469393] WARNING: ./include/linux/rmap.h:351 at folio_add_file_rmap_ptes+0x3b8/0x468,
>>> CPU#90: stress-ng-mlock/9430
>>> [  734.469551]  folio_add_file_rmap_ptes+0x3b8/0x468 (P)
>>> [  734.469555]  set_pte_range+0xd8/0x2f8
>>> [  734.469566]  filemap_map_folio_range+0x190/0x400
>>> [  734.469579]  filemap_map_pages+0x348/0x638
>>> [  734.469583]  do_fault_around+0x140/0x198
>>> ......
>>> [  734.469640]  el0t_64_sync+0x184/0x188
>>> "
>>>
>>> The code that triggers the warning is: "VM_WARN_ON_FOLIO(page_folio(page + nr_pages - 1) != folio, folio)",
>>> which indicates that set_pte_range() tried to map beyond the large folio’s
>>> size.
>>>
>>> By adding more debug information, I found that 'nr_pages' had overflowed in
>>> filemap_map_pages(), causing set_pte_range() to establish mappings for a range
>>> exceeding the folio size, potentially corrupting fields of pages that do not
>>> belong to this folio (e.g., page->_mapcount).
>>>
>>> After above analysis, I think the possible race is as follows:
>>>
>>> CPU 0                                                  CPU 1
>>> filemap_map_pages()                                   ext4_setattr()
>>>     //get and lock folio with old inode->i_size
>>>     next_uptodate_folio()
>>>
>>>                                                            .......
>>>                                                            //shrink the inode->i_size
>>>                                                            i_size_write(inode, attr->ia_size);
>>>
>>>     //calculate the end_pgoff with the new inode->i_size
>>>     file_end = DIV_ROUND_UP(i_size_read(mapping->host), PAGE_SIZE) - 1;
>>>     end_pgoff = min(end_pgoff, file_end);
>>>
>>>     ......
>>>     //nr_pages can be overflowed, cause xas.xa_index > end_pgoff
>>>     end = folio_next_index(folio) - 1;
>>>     nr_pages = min(end, end_pgoff) - xas.xa_index + 1;
>>>
>>>     ......
>>>     //map large folio
>>>     filemap_map_folio_range()
>>>                                                            ......
>>>                                                            //truncate folios
>>>                                                            truncate_pagecache(inode, inode->i_size);
>>>
>>> To fix this issue, move the 'end_pgoff' calculation before next_uptodate_folio(),
>>> so the retrieved folio stays consistent with the file end to avoid 'nr_pages'
>>> calculation overflow. After this patch, the crash issue is gone.
>>>
>>> Fixes: 743a2753a02e ("filemap: cap PTE range to be created to allowed zero fill in folio_map_range()")
>>> Reported-by: Yuanhe Shu <xiangzao@linux.alibaba.com>
>>> Tested-by: Yuanhe Shu <xiangzao@linux.alibaba.com>
>>> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
>>> ---
>>>   mm/filemap.c | 6 +++---
>>>   1 file changed, 3 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/mm/filemap.c b/mm/filemap.c
>>> index bc6775084744..923d28e59642 100644
>>> --- a/mm/filemap.c
>>> +++ b/mm/filemap.c
>>> @@ -3879,14 +3879,14 @@ vm_fault_t filemap_map_pages(struct vm_fault *vmf,
>>>   	unsigned int nr_pages = 0, folio_type;
>>>   	unsigned short mmap_miss = 0, mmap_miss_saved;
>>>   
>>> +	file_end = DIV_ROUND_UP(i_size_read(mapping->host), PAGE_SIZE) - 1;
>>> +	end_pgoff = min(end_pgoff, file_end);
>>> +
>>>   	rcu_read_lock();
>>>   	folio = next_uptodate_folio(&xas, mapping, end_pgoff);
>>>   	if (!folio)
>>>   		goto out;
>>>   
>>> -	file_end = DIV_ROUND_UP(i_size_read(mapping->host), PAGE_SIZE) - 1;
>>> -	end_pgoff = min(end_pgoff, file_end);
>>> -
>>>   	/*
>>>   	 * Do not allow to map with PMD across i_size to preserve
>>>   	 * SIGBUS semantics.
>>
>> I am wondering whether something similar can happen in the do-while loop
>> below this code. We can retrieve a folio from next_uptodate_folio, and
>> then a massive truncate happens and we end up mapping a large folio
>> into the pagetables beyong i_size, violating SIGBUS semantics. (truncation
>> may back-off seeing the locked folio/increased refcount in filemap_map_pages)
> 
> Read the bracket text as - (truncation may fail to unmap this folio seeing
> it locked or with elevated refcount, therefore the illegal mapping stays
> permanent)

IMHO, the truncate_pagecache() will call unmap_mapping_range() twice, 
and the folio lock and refcount will not block unmap_mapping_range() to 
unmap the folio's mapping (only hold ptl lock).

So the truncate_pagecache() can still truncate large folios beyond i_size.